Portion of a mask frame



FIG. 1 is a front perspective view from the top and left side of aportion of a mask frame according to an embodiment of my design;

FIG. 2 is a front perspective view from the bottom and left sidethereof;

FIG. 3 is a front view thereof;

FIG. 4 is a rear view thereof;

FIG. 5 is a left side view thereof;

FIG. 6 is a right side view thereof; and,

FIG. 7 is a top view thereof.

Where included, broken lines showing environmental structure are forillustrative purposes only and form no part of the claimed design.Dotted lines indicate the border between claimed and unclaimed subjectmatter and form no part of the claimed design.

The ornamental design for a portion of a mask frame, as shown anddescribed.